Left Atrium

          As blood leaves the lungs, it returns to the heart through the pulmonary veins. Blood flow from the veins is deposited into the left atrium where it passes through the mitral valve into the left ventricle.
          The left atrium is one of the most structurally complex chambers in the heart. There are usually four pulmonary veins that connect to the posterior wall of this chamber. The area where the veins connect to the atrium is a region of intertwined venous and heart muscle tissues. Many physicians feel that stretching of the tissues where the veins connect to the atrium that happens over time may be a contributing factor to the rhythm atrial fibrillation.
          Beyond the veins, the left atrium itself has many similarities to the right atrium. Below is a list of additional structures that can be found in the left atrium.